Vital Sign Detection Method and Measurement Device

ABSTRACT

A vital sign measurement device includes an occluding device, a motion sensor, and an output unit. The occluding device is adapted to be placed against an anatomical location of a subject, within which is an artery, and to apply a pressure to the anatomical location of the subject to occlude the artery. The motion sensor is positioned with respect to the occluding device to sense movement corresponding to an arterial pulse when the occluding device occludes the anatomical location of the subject. The motion sensor includes a sensor pad positioned for placement against an anatomical location of a subject and to move in response to an arterial pulse. The output unit receives, from the motion sensor, an input indicative of the amount of movement of the sensor pad and generates, using the input, a measure of the vital sign.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/752,724, filed May 23, 2007, which is herein incorporated byreference in its entirety. This application also claims the benefit ofand priority to U.S. Provisional Patent Application Ser. No. 60/802,810,filed on May 24, 2006, U.S. Provisional Patent Application Ser. No.60/874,665, filed on Dec. 13, 2006, and U.S. Provisional PatentApplication Ser. No. 60/898,269, filed on Jan. 31, 2007, all of whichare herein incorporated by reference in their entirety.

TECHNICAL FIELD

This invention relates to detecting vital signs, and more particularlyto a vital sign measurement device.

BACKGROUND

Blood pressure refers to the force exerted by circulating blood on thewalls of blood vessels and constitutes one of the principal vital signs.The systolic pressure is the peak pressure in the arteries, which occursnear the beginning of the cardiac cycle. The diastolic pressure is thelowest pressure, which is at the resting phase of the cardiac cycle. Theaverage pressure throughout the cardiac cycle is reported as the meanarterial pressure. The pulse pressure reflects the difference betweenthe maximum and minimum pressures measured.

Blood pressures can be measured invasively (by penetrating the skin andmeasuring inside the blood vessels) or non-invasively. The former isusually restricted to a hospital setting. The non-invasive auscultatoryand oscillometric methods are simpler and quicker than invasive methods,have less complications, and are less unpleasant and less painful forthe patient. Non-invasive measurement methods are more commonly used forroutine examinations and monitoring.

The auscultatory method typically uses a stethoscope and asphygmomanometer. An inflatable cuff is placed around the upper arm atroughly the same vertical height as the heart and pneumaticallyconnected to a mercury manometer or aneroid gauge. The mercury manometermeasures the height of a column of mercury, giving an absolute cuffpressure measurement without need for calibration and consequently notsubject to the errors and drift of calibration which affect otherpressure gauges. The cuff is inflated manually by repeatedly squeezing arubber bulb until the brachial artery is completely occluded. Whilelistening with the stethoscope over the brachial artery distal to thepressurized cuff, the examiner slowly releases the pressure in the cuff.When blood just starts to flow in the artery, the turbulent flow createsa “whooshing” or pounding sound (first Korotkoff sounds). The pressureat which this sound is first heard is the systolic blood pressure. Thecuff pressure is further released until no sound can be heard (fifthKorotkoff sound), at the diastolic blood pressure.

Oscillometric methods are sometimes used for continuous monitoring andsometimes for making a single measurement. The equipment is functionallysimilar to that of the auscultatory method but does not rely on the useof a stethoscope and an examiner's ear. Instead, the detection means isa pressure sensor that is pneumatically connected to the cuff andregisters the (relatively small) oscillations in cuff pressure that aresynchronous with the arterial pressure waveform. The first oscillationin cuff pressure does not occur at the systolic pressure, but at a cuffpressure substantially above systolic pressure. The cuff is initiallyinflated to a pressure in excess of the systolic blood pressure. Thecuff pressure is then gradually reduced. The values of systolic anddiastolic pressure are calculated from the different oscillationamplitudes that occur at various cuff pressures by the use of analgorithm. Algorithms used to calculate systolic and diastolic pressureoften use experimentally obtained coefficients aimed at matching theoscillometric results to results obtained by using the auscultatorymethod as well as possible.

SUMMARY

In some aspects, a vital sign measurement device includes an occludingdevice, a motion sensor, and an output unit. The occluding device isadapted to be placed against an anatomical location of a subject, withinwhich is an artery, and to apply a pressure to the anatomical locationof the subject to occlude the artery. The motion sensor is positionedwith respect to the occluding device to sense movement corresponding toan arterial pulse when the occluding device occludes the anatomicallocation of the subject. The motion sensor includes a sensor padpositioned for placement against an anatomical location of a subject andto move in response to an arterial pulse. The output unit receives, fromthe motion sensor, an input indicative of the amount of movement of thesensing pad and generates, using the input, a measure of the vital sign.

In some implementations, the occluding device can be an inflatable cuff.In some implementations, the vital sign measurement device can include apressure sensor to detect a pressure applied to the anatomical locationby the occluding device. In some embodiments, output unit can receive,from the pressure sensor, a pressure input indicative of the pressureapplied to the anatomical location and can generate the vital sign usingthe input from the motion sensor and the pressure input.

In some implementations, the anatomical location of the subject the bodycan be an upper arm, and the occluding device can be configured so thatthe motion sensor is positionable to sense movement due to a pulse of abrachial artery. In other implementations, the anatomical location ofthe subject can be a wrist, and the occluding device can be configuredso that the motion sensor is positionable to sense movement due to apulse of a radial artery. In other implementations, the anatomicallocation of the subject can be an ankle, and the occluding device can beconfigured so that the motion sensor is positionable to sense movementdue to a pulse of one or more arteries in the ankle.

In some implementations, the vital sign measurement device can includean optical sensing system including an optical source, an opticalrefractor, and an optical detector. The optical sensing system can sensean amount of movement from the movement, bending, or compression of atleast one portion of the optical sensing system relative to otherportions of the optical sensing system resulting in a change in anoptical signal received by the optical detector.

In some implementations, the motion sensor can include a shaft connectedto the sensing pad and a solenoid. The shaft can move through thesolenoid to create an electrical signal proportional to the movement ofthe shaft in response to the movement of the sensing pad. In someimplementations, the motion sensor can include a potentiometer to detectan amount of movement of the sensor pad.

In some implementations, the motion sensor can include a return elementattached to the sensor pad to counter a force from the arterial pulseand to return the sensor pad to an initial state after the arterialpulse. In some embodiments, the return element can include a spring. Insome implementations, the motion sensor can include a strain gaugeadapted to detect an amount of strain in the spring.

In some implementations, the motion sensor can be adapted such that anapplied pressure of 150 mmHg will displace the sensor pad by at least 1mm from a resting state. In some implementations, the motion sensor canbe adapted such that an applied pressure of 150 mmHg will displace thesensor pad by at least 2 mm from the resting state.

In some implementations, the motion sensor further comprises a housing.In some implementations, the motion sensor can be adapted such than anupper surface of the sensor pad can become approximately flush with anupper surface of the housing when a pressure of between 80 and 150 mmHgis applied to the sensor pad. In some implementations, the motion sensorcan be adapted such than an upper surface of the sensor pad can becomeapproximately flush with an upper surface of the housing when a pressureof between 100 and 130 mmHg is applied to the sensor pad.

In some implementations, the vital sign can be at least one of a heartrate, an arterial pulse waveform, a systolic blood pressure, a diastolicblood pressure, a mean arterial blood pressure, a pulse pressure, and anarterial compliance.

In some implementations, the vital sign measurement device can furtherinclude a display to depict a vital sign measurement generated by theoutput unit. In some implementations, the vital sign measurement devicecan further include an alarm system to produce a human detectable signalwhen a vital sign measurement generated by the output unit meets apredetermined criteria.

In some implementations, the motion sensor and the output unit can beadapted to sense a pulse amplitude of the arterial pulse from thedisplacement of the sensor pad.

In some aspects, a method of measuring a vital sign of a subjectincludes placing an occluding device against an anatomical location of asubject and occluding an artery within the anatomical location, reducingthe pressure applied to the anatomical location of the subject, sensingmovement of a sensor pad corresponding to at least one arterial pulse,and generating a measure of the vital sign. Generating a measure of thevital sign includes using an input indicative of the amount of sensedmovement of the sensor pad. Occluding the artery includes applying apressure to the anatomical location of the subject with the occludingdevice. The occluding device holds the motion sensor having the sensorpad.

The details of one or more implementations of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts one implementation of the vital sign measurement device.

FIGS. 2A, 2B, and 2C depict various implementations of the vital signmeasurement device positioned on an upper arm, and showing threedifferent levels of cuff pressure relative to arterial systolicpressure.

FIG. 3 depicts an implementation of a vital sign measurement devicehaving a occluding device with an inflatable bladder.

FIG. 4 depicts a series of pulses during deflation of a cuff detected bya pressure sensor pneumatically coupled to the cuff compared tosimultaneously obtained pulses detected by an motion sensor held by aoccluding device.

FIGS. 5A, 5B, and 5C depict an implementation of a motion sensorcontaining the components of an optical motion sensor system.

FIGS. 6A, 6B, and 6C depict an implementation of a motion sensorcontaining the components of an optical motion sensor system.

FIGS. 7A and 7B depict a speckle pattern produced by an optical sourcedevice including an optical source and a waveguide.

FIGS. 8A and 8B depict a speckle pattern produced by an optical sourcedevice including an optical source and a diffuser.

FIGS. 9A, 9B, and 9C depict implementations of a motion sensor having anoptical sensing system including a spatial optical occluder.

FIGS. 10A, 10B, and 10C depict implementations of a motion sensor havingan optical sensing system including an optical detector with a pluralityof optical detection regions.

FIGS. 11A, 11B, and 11C depict speckle patterns produced by variousimplementations of vital sign measurement device.

FIG. 12 depicts an electrical signal produced by an optical detectorreceiving a portion of a speckle pattern modulated by an arterial pulse.

FIG. 13 depicts an implementation of an optical detector having aplurality of optical detection regions each producing electricalsignals.

FIGS. 14A, 14B, and 14C depict implementations of the differentanalytical methods used to determine one or more vital signs by theoutput unit.

FIGS. 15A and 15B depict implementations of motion sensors usingnon-optical motion sensing techniques.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

As shown in FIG. 1, a vital sign measurement device can include anoccluding device 102, a motion sensor 104, and an output unit 106. Anoutput from the motion sensor 104 can be used to determine themeasurement of a vital sign. The occluding device 102 can be placedagainst an anatomical location of a subject 112, within which is anartery 118. The motion sensor 104 can be positioned to sense movementcorresponding to an arterial pulse when the occluding device 102 isplaced against the anatomical location of the subject 112. In someimplementations, the motion sensor 104 can be an motion sensor includean optical source 202, an optical refractor 212, 214, or 216 and anoptical detector 240, all of which can be held by the occluding device102. An output unit 106 can receive input from the motion sensor 104that is indicative of movement corresponding to an arterial pulse andcan generate a measure of a vital sign. The motion sensor 104 can sensean arterial pulse from the movement of a sensor pad 232.

For example, a vital sign can include a heart rate, an arterial pulsewaveform, a systolic blood pressure, a diastolic blood pressure, a meanarterial blood pressure, a pulse pressure, and/or a measurement ofarterial compliance. In some implementations, the vital signs can bedetermined from the timing of arterial pulses, the amplitude and/ormagnitude of arterial pulses, or from arterial pulse waveforms. In someimplementations, the vital signs can be determined from output receivedfrom the motion sensor 104 alone or in combination with other data(e.g., data regarding the pressure applied by the occluding device). Forexample, in some implementations, a heart rate can be determined fromthe output received from the motion sensor 104 alone.

Occluding Device

The occluding device 102 can be any structure adapted to apply anoccluding pressure to an anatomical location of a subject 112 and tohold and position the motion sensor 104 or a portion thereof adjacent toan anatomical location of a subject 112 such that the motion sensor 104can detect an arterial pulse. For example, the occluding device 102 canbe an adhesive bandage or a cuff (e.g., an elastic cuff or an inflatablecuff). In some implementations, the occluding device 102 can be aninflatable cuff 120 having an inflatable bladder 122. The bladder 122can be pneumatically connected to a pump 124 via a hose 116. Forexample, a pneumatically inflatable cuff can be inflated (e.g., via apump 124) and deflated (e.g., via a valve 126) to adjust the pressureapplied to a portion of a subject's body 112.

The occluding device 102 can be applied to any portion of a subject'sbody. In some implementations, the occluding device 102 is sized andarranged for placement at an anatomical location of a subject's bodyadjacent to a predetermined artery 118 of the subject. As shown in FIGS.2A, 2B, and 2C, the occluding device 102 can be positioned on an upperarm (above a subject's elbow) so that the motion sensor 104 can sensemovement corresponding to an arterial pulse in the brachial artery 118.The occluding device 102 can also be adapted for placement on the wristso that the motion sensor 104 can sense movement corresponding to anarterial pulse in the radial artery. The occluding device 102 can alsobe positioned on a leg (e.g., at the ankle to detect pulses in anartery), the neck, or any other part of the body where an arterial pulsecan be detected.

As shown in FIGS. 2A, 2B, and 2C, the motion sensor 104 can bepositioned proximal to the midpoint of the occluding device 102 (asshown in FIG. 2A), at the mid point of the occluding device 102 (asshown in FIGS. 2B and 2C), or distal to the mid point of the occludingdevice 102 (not shown). The placement of the motion sensor 104 withinthe occluding device 102 can impact the data obtained. In someimplementations, a pressure applied to an artery lying below the surfaceof an anatomical location can be non-uniform. For example, although anoccluding device 102 can apply a uniform pressure, the pressuretransmitted through the layers of tissue can result in a non-uniformpressure against an artery lying some distance below the surface. Insome implementations, the pressure applied to an artery lying somedistance below the skin by an inflatable cuff can be greatest at thecuff midline and less at the cuff margins. The location of the motionsensor 104 relative to the occluding device 102 can be fixed to optimizethe sensitivity to selected features of the arterial pulse. In someimplementations, the motion sensor 104 can be located at the midline ofthe cuff such that it is not responsive to pulsatile enlargement of thearterial segment under the proximal part of the cuff when the cuffpressure exceeds systolic pressure, thereby allowing a precisedetermination of the systolic pressure when the midsection of thearterial segment opens.

In other implementations, the motion sensor 104 can be located near thedistal margin of the cuff such that it is responsive specifically to thepulsatile arterial dimension changes at that location. Accordingly, theunique features of the arterial pulse waveform at diastolic pressure, ata distal position can be identified, and effects of arterial compliancein more distal arteries can be detected. Outward flexing of the skin atthe midline of the cuff, and also distal to the midline, occurs duringsystole when the cuff pressure is below systolic pressure. At cuffpressures exceeding systolic blood pressure, the arterial oscillationsare limited to the proximal area of the cuff, as discussed above. Insome implementations, the motion sensor 104 can be located on a bodyfixation device 102 separate from a pressure imparting device adapted tobe placed against a second anatomical location of a subject proximal tothe anatomical location of the occluding device 102 to allow forarterial pulse detection by the motion sensor 104 at a position distalto and separated from the pressure imparting device. For example, thepressure imparting device can be an inflatable cuff. In someimplementations, both the pressure imparting device and the bodyfixation device 102 can be inflatable cuffs.

FIG. 2A depicts a occluding device 102 imparting a pressure on the armexceeding arterial systolic pressure of the brachial artery sufficientto result in a minimal arterial opening under the leading edge of theoccluding device 102 at systole. The amount of pressure imparted againstthe occluding device 102 will pulsate slightly due to the arterialexpansion at the leading edge during an arterial pulse. No arterialopening occurs at the positioning of the motion sensor 104, andtherefore the motion sensor 104 does not produce a signal indicative ofmovement. A movement signal, however, will occur at a higher pressure ifthe motion sensor 104 is located at a position proximal to the midpointof the occluding device 102 than if it is located at the midpoint of theoccluding device 102.

FIG. 2B depicts a occluding device 102 imparting a pressure slightlyexceeding arterial systolic pressure, such that the arterial opening 118extends nearly to the midpoint of the occluding device 102 at systole.The oscillation in pressure imparted against the occluding device 102during an arterial pulse pressure would be much larger than in the caseof FIG. 2A, as the arterial expansion occurs over nearly half of thesegment located within the occluding device. Nevertheless, no arterialopening occurs at the occluding device 102 midpoint, and therefore themotion sensor 104 does not produce a signal indicative of movement.

FIG. 2C depicts a occluding device 102 imparting a pressure belowarterial systolic pressure, such that the entire artery segment 118opens momentarily at systole. The oscillations in pressure impartedagainst the occluding device 102 during an arterial pulse will be evengreater in amplitude. The arterial opening at the location under themotion sensor causes the motion sensor to detect movement of an sensorpad 232.

FIG. 3 depicts one implementation of a occluding device 102. Theoccluding device can be an inflatable cuff 120 having an inflatablebladder 122. The inflatable cuff 120 can be adapted to be wrapped aroundthe upper arm of a subject to allow the motion sensor 104 to detectmovement due to arterial pulses from the brachial artery. The componentsof the motion sensor 104 can be packaged within a housing 200 located atthe at the midpoint 134 of the cuff 120. The cuff 120 can include hookand loop fasteners 132 (e.g., Velcrot) or other fastening devices, whichcan be used to secure the cuff 120 around a limb of a subject. The cuff120 can be wrapped around a subject's limb and the bladder 122 inflatedto impart a pressure on the limb. The bladder 122 can be connected to apump 124 by a hose 116. The bladder 122 can also be attached to a valve126 which can control the deflation of the bladder 122. The pressure inthe bladder 122 can be measured with a pressure transducer 128. Thepressure transducer 128 can be located in the bladder, as shown, or canbe pneumatically connected to the bladder 122 (e.g., via the hose 116).

The top portion of FIG. 4 depicts pressure pulses sensed in a occludingdevice 102 imparted by the series of arterial pulses as the impartedpressure by the occluding device 102 is decreased from a pressureexceeding systolic blood pressure of a subject to a pressure belowdiastolic blood pressure of a subject. The bottom portion of FIG. 4depicts pulses detected by a motion sensor 104 placed at the midpoint ofan occluding device 102 as the pressure imparted by the occluding device102 is decreased from a pressure exceeding systolic blood pressure of asubject to a pressure below diastolic blood pressure of a subject. Asshown, the motion sensor 104 does not detect any pulses until theimparted pressure is at or below systolic blood pressure. In someimplementations, this can allow for an accurate determination ofsystolic blood pressure.

Output Unit

Detected movements from the motion sensor 104 can be transmitted viaelectrical wires 108 to a display device 114. In some implementations,as shown in FIG. 3, electrical wires 108 can connect a pressuretransducer 128 to a display device 114. An output unit 106 (not shown inFIG. 3) can be part of the display unit 114, can be within the opticalsensor housing 200, can be in another portion of the cuff assembly, orcan be remotely located and in communication with the motion sensor 104via wireless transmissions. In some implementations, the output unit 106can transmit vital sign measurements via wireless transmission. In someimplementations, the motion sensor 104 can transmit data regarding theamount of movement to an output unit 106 via wireless transmission. Insome implementations, the motion sensor 104 can send other outputrepresentative of an amount of movement to the output unit 106 (e.g., anamount of light received by a photo detector in an optical motion sensorsystem). The output unit 106 can comprise a processor to determine thevital sign from signals from the motion sensor 104 with or without otherdata. In some implementations, as shown in FIG. 1, the output unit caninclude a display to depict the vital sign. In some implementations, theoutput unit can include an alarm system to produce a human detectablesignal when a vital sign measurement generated by the output unit meetsa predetermined criteria. For example, the output unit can be adapted tocreate a visual or audio alarm to alert a user that a detected vitalsign is outside of a predetermined range. The output unit 106 canperform a number of data processing steps, calculations, or estimatingfunctions, some of which are discussed below.

Motion Sensor

The motion sensor 104 can include a motion sensing system adapted todetect localized motion associated with an arterial pulse when theoccluding device is placed against the anatomical location of thesubject. As shown in FIGS. 5A, 5B, 5C, 6A, 6B, and 6C, the motion sensor104 can be include a housing 200 and a sensor pad 232 adapted to move inresponse to an arterial pulse. The motion sensor 104 can detect anamount of movement of the sensor pad 232 though a number of motionsensing techniques. In some implementations, the motion sensor 104 canuse optical techniques to detect an amount of movement of the sensor pad232. In some implementations, various electrical and/or mechanicaltechniques can be used to detect an amount of movement of the sensorpad. Some of these motion sensing methods are discussed in furtherdetail below.

The motion sensor can also include a return element 234 attached to thesensor pad to counter a force from the arterial pulse and to return thesensor pad to an initial state after the arterial pulse. For example,the return element can be a spring. The return element can provide asufficient force such that an applied pressure of 150 mmHg will displacethe sensor pad by at least 1 mm from a resting state (e.g., a statewhere nothing except air pressure is pressing against the sensor pad).In some implementations, the motion sensor can be adapted such that anapplied pressure of 150 mmHg will displace the sensor pad by at least 2mm from the resting state. In some implementations, the motion sensorcan be adapted such that an applied pressure of between 80 and 150 mmHg(e.g., between 100 and 130 mmHg) can render an upper surface of thesensor pad approximately flush with an upper surface of the housing. Insome implementations, the sensor pad 232 can be nearly flush with thehousing when placed against the anatomical location of a patient by theoccluding device 102 with the occluding device providing a pressure tothe anatomical location exceeding systolic pressure. In someimplementations, the upper surface of the housing can be approximatelyflush with an inner surface of the occluding device 102.

FIGS. 5A, 5B, 5C, 6A, 6B, and 6C show examples of miniaturized opticalmotion sensors that can be placed against a subject's skin to sensearterial pulses. The optical sensor housing 200, as shown, includes asensor pad 232 and a return element 234 attached to the sensor pad 232.In some implementations, the sensor housing 200 can have a width ofbetween 0.7 and 1.3 inches (e.g., about 1 inch), a length of between 1.5and 2.2 inches (e.g., about 1.7 inches), and a thickness of between 0.3and 0.9 inches (e.g., about 0.6 inches). The sensor pad 232, when in aresting state, can extend out of the optical sensor housing 200. Forexample, the sensor pad 232 can extend out of the optical sensor housing200 by at least 0.1 inch (e.g., between 0.1 and 0.3 inches). As shown,the sensor pad 232 extends out from the sensor housing 200 by 0.161inches. The return element 234 (e.g., a spring) can provide a returnforce such that an applied pressure of between 80 and 150 mmHg (e.g.,between 100 and 130 mmHg) can displace the sensor pad 232 such that thesensor pad is approximately flush with the upper surface of the housing200. The sensor pad 232 can have any shape. The sensor pad 232 can havea diameter of at least 0.3 inches, for example between 0.3 and 0.8inches (e.g., about 0.6 inches). In some implementations, for example asshown in FIG. 6C, the sensor pad 232 can be attached to the returnelement 234 by a hinge 236 that allows for the back and forth motion ofthe sensor pad 232. In some implementations, as shown in FIG. 6C, thesensor pad 232 can have an inclined upper surface.

The sensor pad 232 can also be positioned within a cutout 252. Thespacing between the cutout 252 and the sensor pad 232 can impact theamount of movement of the sensor pad 232 allowed by the sensor housing200 due to arterial pulses. The spacing between the cutout 252 and thesensor pad 232 can be about 0.1 inches.

The sensor pad 232 can be a button that can move when a differentialpressure is applied to it. When this apparatus is applied against ananatomical location, where the artery within the anatomical location iscontinuously occluded by the occluding device 102, by way of itsattachment to the occluding device 102, there is little or no movementin connection with pulsatile tensioning of the occluding device 102(e.g., an inflatable cuff). Therefore little or no motion is detectedwhen the occluding device 102 applies a pressure above systolicpressure. When the pressure applied by the occluding device 102 isreduced such that the artery segment under the occluding device 102 ismomentarily opened at systole (the highest pressure during the cardiaccycle), an outward displacement to the “incompressible” tissue occursthat is transmitted to the motion sensing member. Therefore there can bean abrupt transition when the occluding device 102 is at systolicpressure. Little or no motion detection occurs above systolic pressure,but beginning at systolic pressure and continuing as the pressureapplied by the occluding device 102 is further decreased, the arterialopening imparts outward tissue displacement that is detectable by themotion sensor.

The motion sensor 104 detects an amount of motion of the sensor pad 232,rather than merely a pressure applied to the sensor pad 232. Forexample, a surface pressure sensor (e.g., a piezoresistive type pressuresensor) can detect changes in pressure due to an arterial pulse evenwhen the pressure applied to the anatomical location by the occludingdevice 102 exceeds systolic pressure. At high cuff pressure (abovesystolic pressure) the artery proximal to the occluding device 102(e.g., a inflatable cuff) can impart a pulsatile impact to theanatomical location delivered through the tissue, which causes apulsatile pressure increase within the occluding device 102. This effectcauses a pulsatile tensioning of the occluding device 102, which wouldbe detected by a surface pressure sensor attached to the inside surfaceof the occluding device 102, even though there is no cuff contractionbecause the tissue is essentially “incompressible” and the artery iscontinuously occluded in the area underneath the pressure sensor. Asignal of an amount of pressure applied by the occluding device 102(i.e., a bladder pressure sensor) and the surface pressure sensor willbe similar above and below systolic pressure because the effect of theopening of the artery to allow blood flow to occur is smaller than theeffect of the pulsatile impact to the cuff described above. In contrast,a motion sensor 104, has little to no response to the tensioning of thecuff at high cuff pressures. The motion sensor 104 does not detectsignificant motion due to arterial pulses at pressures above systolicpressure. Accordingly, the use of motion sensor 104 can more accuratelyindicate the systolic blood pressure than a pressure sensor.Furthermore, no separate accurate blood pressure measurement is neededfor calibration or establishment of a baseline when using a motionsensor 104.

As described below, a variety of motion sensing techniques andanalytical methods can be used to detect the amount of motion of thesensor pad 232 due to an arterial pulse.

Optical Motion Sensing Techniques

In some implementations, the motion sensor can include an opticalsensing system including an optical source 202, an optical refractor212, 214, or 216 and an optical detector 240, all of which can be heldby the occluding device 102 and move with movement of the occludingdevice 102. The optical sensing system 104 can detect motioncorresponding to an arterial pulse when the occluding device is placedagainst the anatomical location of the subject.

In some implementations, the optical sensing system can include anoptical source 202 optically coupled to an optical refractor 212, 214,or 216, such that light waves travel from the optical source 202 to theoptical refractor 212, 214, or 216. The optical source 202 can be acoherent light source, for example a laser. In some implementations, anLED can be used as the optical source 202.

In some implementations, the optical refractor can be an opticalwaveguide 212, a diffuser 214, a mirror with surface imperfections 216,or another refractive material. The movement, bending, or compression ofthe optical refractor 212, 214, or 216 can alter the path taken byoptical waves 218 traveling through the optical waveguide 212, throughthe diffuser 214, or refracting off of the mirror 216, thus causing theamount of optical energy (e.g., light) received by the optical detector240 or 242 to change. Likewise, the movement of the optical source 202or the optical detector 240 or 242 can result in changes to the amountof optical energy (e.g., light) received by the optical detector 240 or242. By monitoring the changes in the amount of received optical energy,an arterial pulse can be characterized, which can be used to determine avital sign. For example, the amplitude of the pulse can be determined,or the waveform shape of the pulse can be determined.

In some implementations, the optical detector 240 or 242 can be a PINdiode photodetector, a CCD (Charge-Coupled Device) detector, or a CMOS(Complementary Metal-Oxide-Semiconductor) detector. In someimplementations, the optical sensing system can include one or moreoptical detectors 240 or 242. For example, in some implementations, aseries of optical detectors can each receive optical energy refracted bythe optical refractor 212, 214, or 216. In some implementations, anoptical detector 242 can include a plurality of optical detectionregions. For example, CCD and CMOS detectors can be configured to allowfor the detection of the amount of optical energy received by aplurality of discrete detection regions or can be configured to output asignal indicating the total amount of optical energy received by the CCDor CMOS detector.

In some implementations, such as those discussed below, the opticalsource 202 and the optical refractor 212, 214, or 216 are arranged toproduce a speckle pattern. In some implementations, the compressionand/or bending of a compressible or flexible optical waveguide canresult in a change in the total amount of light exiting the opticalwaveguide or a change in a speckle pattern.

Referring again to FIGS. 5A, 5B, 5C, 6A, 6B, and 6C, the optical sensorhousing 200 also includes a optical source 202, an optical refractor212, 214, or 216, a optical detector 240 or 242, and wires 108 from theoptical detector 240. In some implementations, the optical sensorhousing 200 can also include additional elements, such as a spatialoptical occluder 222 (e.g., a pin hole aperture) between the opticalrefractor 212, 214, or 216 and the optical detector 240 or 242, asdepicted in FIG. 5C. The sensor pad 232 can be attached to or otherwisepositioned to cause the relative movement of the optical source 202, theoptical refractor 212, 214, or 216, any spatial optical occluder 222 ifused, the optical detector 240, or a combination thereof. As shown inFIG. 6C, the sensor pad 232 can include a pressing portion 238 adaptedto cause the bending, compression, or movement of an optical waveguide212. In some implementations, such as shown in FIG. 5C, the returnelement 234 (e.g., spring) can be attached to an optical source 202,such that the modulation of the return element 234 causes the movementof the optical source 202 while the optical refractor 214 remainsstationary. The return element 234 can have a length of at least 0.6inches, for example between 0.6 inches and 1.8 inches (e.g., 1.1inches). Various other configurations can allow for the modulation ofthe return element 234 to result in the relative movement of the opticalsource 202 and the optical refractor 212, 214, or 216.

Wires 108 can transmit data from the motion sensor 104 to an output unit106, as discussed above. In some implementations, the output unit can beincluded within the housing 200 and wires 108 can transmit vital signdata to devices outside of the housing 200. In some implementations (notshown), the motion sensor 104 can transmit data from a housing 200 bywireless transmission.

Speckle Pattern

FIGS. 7A, 7B, 8A, and 8B depict the basic principle of speckle patternmodulation. A optical source 202 can be optically coupled to an opticalrefractor 212, 214, or 216, such that optical waves 218 travels from theoptical source 202 to the optical refractor 212, 214, or 216. Theoptical source 202 can provide coherent light. The optical source 202,such as a laser, can be used to illuminate the optical refractor 212,214, or 216 to create a “speckle pattern” 260, so-called because theoptical effect is the appearance of speckles 262 in the far fieldillumination. For example, the optical refractor can be opticalwaveguide 212, a diffuser 214, a mirror with surface imperfections 216(e.g., as shown in FIGS. 9C and 10C), or another refractive materialcapable of forming a speckle pattern 260. The refraction can causespatial variations in the transmitted optical waves 218 which appear asregions of darkness in a background of light. These dark regions, orspeckles 262, can be of characteristic, but random, shape and size,determined by the refractive characteristics of the optical refractor212, 214, or 216. The optical waves 218 (only a few of which areillustrated) illuminating the optical refractor 212, 214, or 216 canconstructively interfere to form a speckle pattern 260 of a series ofspeckles 262. The relative movement, bending, or compression of theoptical refractor 212, 214, or 216 relative to the optical source 202alters the path taken by the optical waves 218 traveling through theoptical refractor 212 or 210 or refracting off of the refractor 310,thus causing the speckle pattern 260 to change. For example, as anoptical refractor 212, 214, or 216 is moved relative to the opticalsource 202, the speckle pattern 260 can seem to twinkle or, in somecases, can seem to rotate. Although the total light traveling throughthe optical refractor 212 or 210 or refracting off of the mirror 216 canremain relatively constant, by monitoring a select detected portion,e.g., 264, of the speckle pattern, changes in the amount of opticalenergy (e.g., light) in a detected portion 264 of the speckle pattern260 can be observed. By monitoring the changes in the amount of light inthe detected portion, e.g., 264, the amount of and/or speed of relativemovement, bending, or compression can be determined.

The detected portion, e.g., 264, can be limited by restricting theportion of the formed speckle pattern 260 allowed to be received by theoptical detector 240 or 242. Restricting the portion of the specklepattern 260 received by a optical detector 240 can be achieved in anumber of ways. For example, as shown in FIGS. 9A, 9B, and 9C, a spatialoptical occluder 222, such as a blocking structure having an opticalaperture formed therein (e.g., a pin hole aperture), can be positionedbetween the optical refractor 212, 214, or 216 and an optical detector240. In some implementations, the detected portion 264 of the specklepattern 260 can be restricted by using an optical detector 240 having asmaller optical energy receiving area than the area of produced specklepattern 260. The optical detector 240 or 242, and any intermediatespatial optical occluder 222 used, can be placed adjacent to the opticalrefractor 212 or 214 to ensure that the optical detector 240 or 242 onlyreceives light from speckles within a predetermined detected portion,e.g., 264. When using a mirror with surface imperfections 216 as theoptical refractor, the spacing of the optical detector 240 and anyintermediate spatial optical occluder used, will determine the size ofthe detected portion 264 and of the produced speckle pattern 260.

The optical source 202 can be a coherent light source, for example alaser.

The optical refractor can be an optical waveguide 212, a diffuser 214,or a mirror having surface imperfections 216, or another refractivematerial capable of forming a speckle pattern 260. In someimplementations, a device can use a combination of multiple and/ordifferent optical elements. For example, an optical waveguide 212 can byused to guide light waves 218 to a diffuser 214.

An optical waveguide 212 can be an optical fiber or any liquid, gel, orsolid that transmits light waves by internal reflection or refraction.In some implementations, the optical waveguide 212 can transmit almost100% of the light by providing almost total internal refraction. Forexample, an optical waveguide 212 can include an optical material withrelatively high index of refraction (n_(h)), surrounded by a materialwith lower index of refraction (n_(l)). In such optical waveguides 212,light is lost only when the light wave reaches the interface between thetwo materials at an angle less than the critical angle (θ_(c)). Thecritical angle (θ_(c)) can be calculated by the following equation.θ_(c)=arcsin (n _(i) /n _(h))In some implementations, the surrounding material with a lowerrefractive index can be air. In some implementations, waveguides canalso be in the form of a hollow tube with a highly reflective innersurface. The inner surfaces can be polished metal.

In some implementations, such as that shown in FIGS. 7A and 7B, anoptical waveguide 212 causes the internal reflection of optical waves218 within the core of the optical waveguide 212. As the opticalwaveguide 212 is moved or bent, the path for each light wave 115 isaltered, resulting in changes in a resulting speckle pattern. In someimplementations, the optical waveguide 212 can be a flexible waveguide.In some implementations, the optical waveguide 212 can be a compressiblewaveguide.

A diffuser 214 can be any device comprised of refractive material thatdiffuses, spreads out, or scatters light in some manner, such as anysemitransparent liquids, gels, or solids; airborne particles; and/orskin or other tissue. For example, a diffuser 214 can includepolyoxymethylene (POM) (e.g., Delrin® acetal resin), white fluoropolymer(e.g., Teflon® fluoropolymer), Polyamide (PA) (Nylon®), or ground orgrayed glass. In some implementations, the diffuser material can havelow optical absorption at the laser wavelength, and can have refractiveproperties that produce sufficient light scattering over a short pathlength to insure that a speckle pattern is generated on the surfaceopposite the laser with suitable speckle size and uniformity. Forexample, the diffuser can include a piece of polyoxymethylene (Delrin®acetal resin) having a thickness of between 0.2 mm and 1 mm (e.g.,between 0.4 and 0.6 mm), such that the optical intensity is not overlydiminished on the exit side but sufficiently thick to effect therequisite light scattering needed to create the speckle pattern 260.

In some implementations, such as that shown in FIGS. 8A and 8B, adiffuser 214 causes the refraction of light waves within the body of thediffuser 214. The refraction of light waves within the diffuser can becaused by variations in refractive index within the diffuser 214 whichresult in random photon scattering. As the diffuser 214 is moved, theareas of the diffuser which cause the refraction of the light waves arealso moved, causing the optical waves 218 to refract differently withinthe diffuser 214, resulting in changes in a resulting speckle pattern260.

In some implementation, such as shown in FIGS. 9C and 10C, the opticalelement can also be a mirror with surface imperfections 216. Theimperfections in the mirror can result in light waves impacting theimperfections to reflect at different angles. The reflection of lightoff of the mirror with imperfections 216 also can result in an opticalpattern 260. The relative movement of the mirror 216 in respect to theoptical source 202 similarly results in changes to the optical pattern260.

In some implementations, the characteristic size and number ofindividual speckles 262 can be controlled. For example, thecharacteristic size and number of individual speckles 262 can becontrolled with an optical waveguide 212 having optimal diameter andrefractive characteristics for the desired speckle 125 features.Illustrated in FIGS. 11A and 11B are the speckle patterns 260 from alaser 202 whose beam is passed through different optical fibers. In FIG.11A, a speckle pattern with relatively few, large speckles 262 is shown,which is formed from an optical waveguide 212 having a small diameterand small index of refraction gradient. In contrast, the speckle pattern260 shown in FIG. 11B with relatively many, small speckles 262 is formedwith an optical waveguide 212 that permits much more opticalinterference because of a larger diameter and larger index of refractiongradient, resulting in a speckle pattern 260 with relatively many, smallspeckles 262.

Similarly, FIG. 11C is a magnification of a speckle pattern 260 formedby passing coherent light through a diffuser 214. The bar in the upperright side of the figure indicates the size of the magnification.

In some implementations, the average speckle size of the sampled portionof a speckle pattern 260 can be at least 10 microns (for example,between 25 and 100 microns).

Sensitivity to the relative movement, bending, or compression of theoptical source and the optical refractor 212, 214, or 216 can beoptimized by properly sizing the detected portion 264 and fixing theseparation of the optical refractor 212, 214, or 216, the opticaldetector 240, and any intervening spatial optical occluder 222 if used.The detected portion 264 can be sized in relation to the average specklesize so as to optimize the amplitude of fluctuations in the electricaloutput of the optical detector 240, which correspond to the modulationof the speckle pattern 260 that is caused by relative movement, bending,or compression of the optical refractor 212, 214, or 216, the opticalsource 202, or the optical detector 240 or 242. For example, by sizingan aperture of a spatial optical occluder 222 to collect only a smallnumber of speckles, such as less than one percent of the speckle pattern260 area, and employing suitable signal processing to the time-varyingoptical detector output, the time derivative of the pulse signal can bemeasured to allow a calculation of a vital sign. In someimplementations, the optical energy receiving portion of the opticaldetector 240 can also have a smaller area than the area of the producedspeckle pattern 260.

In some implementations, the detected portion 264 of the speckle pattern260 can be less than one hundred times the average speckle size, forexample, between 1 and 25 times the average speckle size. In someimplementations, the optical detector 240 can receive up to an averageof 50 speckles, for example between 1 and 5 speckles. For example, a pinhole aperture having a 125 micron diameter can be used to restrict thedetected portion 264 of the speckle pattern 260 received by a opticaldetector 240 or 242.

Analytical Methods for Optical Motion Sensing Techniques

The optical detector 240 or 242 of an optical motion sensor 104 cangenerate an electrical signal 420 indicating the amount of lightreceived. The electrical signal 420 can be a function of time. Theelectrical optical detector signal 420 is analyzed to determine the rateof modulation of the speckle pattern 260. For example, FIG. 12 depicts apossible electrical signal 420 indicating the modulation in an amount ofoptical energy received by an optical detector 240 or 242. As shown inFIG. 12, the amount of light received by the optical detector 240 canoscillate. The oscillation frequency of optical energy received by theoptical detector 240 or 242 can be generally understood as the inverseof the amount of time in which a characteristic change occurs in thenumber or brightness of speckles within the predetermined detectedportion, e.g., 264, which is received by the optical detector 240 or242. A characteristic change occurring in the number of brightness ofspeckles can be generally scaled to represent a characteristic relativemovement, bending, or compression of the optical source and the opticalrefractor. By monitoring the rate of oscillation of the amount of lightreceived by the optical detector 240, the amplitude and/or magnitude ofan arterial pulse can be determined.

In some implementations, the average amount of light received by theoptical detector 240 can vary over time in response to the positioningof the light source relative to the optical refractor 212, 214, or 216and the amount of light received by the optical detector 240 canoscillate about that average amount of light received due to therelative movement of the optical source and the optical refractor.

In some implementations, this low frequency variation in the amount oflight received can be filtered out of the received signal. In someimplementations, high frequency “noise” can also be filtered out. Insome implementations, high and/or low frequency variations in the amountof light received by an optical detector can be filtered out of thesignal from an optical detector 240 or 242 prior to determining a vitalsign from the data. In some implementations, the filtering of the signalcan be performed by an optical waveform prefilter 432.

The output unit 106 can determine the amplitude and/or magnitude of eacharterial pulse to determine one or more vital signs. In someimplementations, the amplitudes and/or magnitudes for a series ofarterial pulses can be determined to determine one or more vital signs.For example, to determine the amplitude and/or magnitude of an arterialpulse from the oscillations of the amount of light received by theoptical detector 240, a differentiating electrical circuit can beapplied to an optical detector 240 output to produce a signalproportional to its time derivative, dE/dt. This time-derivative signalcan increase in proportion to the frequency content of the opticaldetector electrical signal, which is proportional to the rate ofmodulation of the speckle pattern. Each arterial pulse (corresponding toa cardiac cycle), can, for example, characteristically exhibit apressure increase, followed by a pressure decrease, and then a quiescentperiod before the start of the next pulse. The pressure increase cancause the optical source 202 to move or the optical refractor 212, 214,or 216 to move, bend, or compress such that the speckle pattern 260modulates. The modulation rate will increase at the start of the pulseand decrease to zero at the time of maximum pulse pressure (i.e., wherethe pulse wave stops rising, and is about to begin its decline). As thepressure decreases, an opposite movement of the waveguide will occur,again modulating the speckle pattern such that its modulation rateincreases after the maximum pulse pressure and decreases to zero whenthe arterial pulse has ended. FIG. 12 depicts an example of a opticaldetector electrical signal created by an arterial pulse. The signaldE/dt will therefore start at zero, then increase to a maximum, thendecrease to zero, then increase again, and finally decrease to zero, allduring the course of one arterial pulse. The pulse amplitude can be, asa first approximation, proportional to the maximum speckle patternmodulation rate, which in turn can be calculated from the maximum valueof dE/dt, based on the relationship between a sinusoidal function andits derivative, i.e.:dE/dt=d/dt[sin(ωt)]=ω·cos (ωt),whose maximum amplitude is proportional to the maximum modulation rateduring the arterial pulse cycle, or ω_(max).

The signal dE/dt can be analyzed with a real-time spectrum analyzer,such as a digital signal processor (DSP), to determine the maximumfrequency during the arterial pulse cycle. The maximum frequency,ω_(max), occurs at the maximum of dE/dt, and in the same way scales withthe pulse amplitude. The highest dominant frequency, ω_(max) can be usedfor analysis or, if a range of frequencies is present, the first,second, or other moment of the frequency spectrum can be used.

The optical detector 240 output can also be AC coupled and fed into azero-crossing detector, which provides a count of the number of zerocrossing events per unit time (a “zero-crossing rate”) and a total countof zero-crossing events during one arterial pulse (the “zero-crossingcount”). By properly limiting the size of the detected portion 264, theinstantaneous zero-crossing rate is easily shown to be proportional tothe rate of modulation of the speckle pattern 260. An algorithm can beapplied to detect the rise of the zero-crossing rate above zero, andthen to count the number of zero crossings until the zero-crossing ratereturns to zero. A threshold slightly above zero can be used, instead ofa true zero-crossing rate, to account for system “noise.” Alternatively,high frequency noise can be filtered out of a signal from the opticaldetector 240 or 242. The count can be repeated after the zero-crossingrate again rises above zero until its return to zero. This cycle,including two zero-crossing counts, is taken to correspond to onearterial pulse. The two counts, averaged together, can be proportionalto the amplitude of the waveguide oscillatory movement in connectionwith the arterial pulse, and therefore can also be proportional to thearterial pulse amplitude. An algorithm can be applied to thezero-crossing rate that measures the time at which this rate remains atzero between non-zero episodes. In a sequence of arterial pulses, arelatively longer time can occur between the end of one arterial pulseand the onset of the next one. A relatively shorter time can occur atthe maximum pulse pressure, where the pressure stops rising and beginsto decrease, in which the zero-crossing rate can be zero momentarily.

In some implementations, the signal dE/dt can be passed through anintegrating circuit and integrated over the time from its rise abovezero until its return to zero. This time corresponds to the half cycleof the arterial pulse, which can be determined by separately measuring atime-averaged value of dE/dt to determine when it departs from andreturns to zero. The resulting integration can be proportional to theamplitude of the waveguide oscillatory movement, and therefore can alsobe proportional to the arterial pulse amplitude. This integration of thefirst derivative of a subject's position over a specified time periodcan yield a result proportional to the change in position during thespecified time period.

In some implementation, as shown in FIGS. 10A, 10B, and 10C, a pluralityof optical detection regions 244 can be used. These optical detectionregions 244 can be part of an optical detector 242 that contains anumber of discrete optical detection regions 244. For example, opticaldetector 242 can be a CCD (Charge-Coupled Device) or CMOS (ComplementaryMetal-Oxide Semiconductor) detector. Each optical detection region 244can be configured to only receive a restricted portion of a specklepattern 260, for example, as shown in FIGS. 10A, 10B, and 10C. Using aplurality of optical detection regions 244 one can obtain data that morereliably represents the relative amplitudes of a series of pulsepressure waveforms. In some implementations, the output from a pluralityof optical detection regions 244 can each be AC coupled and fed into azero-crossing detector. The electrical signals 420 corresponding to thedifferent optical detection regions 244, as shown, for example, in FIG.13, can be compared at the end of each arterial pulse or at the end ofeach blood pressure measurement cycle to determine which has the highestsignal quality. The quality of an electrical signal 420 can also bedetermined by detecting a zero-crossing count for each signal. Forexample, the electrical signal 420 with the highest count may beconsidered to have the highest signal quality. The differentzero-crossing counts for each of the different detectors (or a subset ofdifferent detectors) can also be averaged for each arterial pulse toproduce a more reliable estimate of the pulse amplitude.

In some implementations, the output from a plurality of opticaldetectors can each be coupled to a differentiating circuit to measuredE/dt. The different values of dE/dt corresponding to the differentdetectors can be compared at the end of each arterial pulse or at theend of each blood pressure measurement cycle to determine which has thehighest signal quality. For example, the one with the highest value ofdE/dt_(max) may be considered to have the highest signal quality. Theplurality of different values of dE/dt corresponding to the differentdetectors (or a subset of different detectors) can also be averaged foreach arterial pulse to produce a more reliable estimate of the pulseamplitude.

In some implementations, a CCD (Charge-Coupled Device) or CMOS(Complementary Metal-Oxide-Semiconductor) detector can be used as eithera single optical detector 240 or as a plurality of optical detectionregions 244. A typical CCD or CMOS detector can have over 1 millionpixels, and those in consumer grade digital cameras may have up to 8million or more pixels in a 1-2 cm rectangular sensor. Each pixel, orseparately addressable sensing region, may function as a separateoptical detection region 244. “Binning” can also be used to effectivelyenlarge the detector sensing areas by combining the outputs of an N×Mgroup of pixels (e.g., 2×2, 2×3, 3×3, etc). In some implementations, thesize of the detected portion 264 for each optical detection region 244can be dynamically adjusted by “binning.” For example, during the lifeof a sensor the optical characteristics of the optical refractor 212,214, or 216 can change and the size of the “binned” group of pixels canbe dynamically adjusted during the life of the optical motion sensor 104to re-optimize the size of the detected portion 264. In someimplementations, each group of pixels acting as a optical detectionregion 244 can have the same or different sizes, which can be optimizeddepending upon the portion of the speckle pattern 260 received by thatgroup of pixels. The use of a CCD or CMOS optical detector 240 or 242can allow for a device without an optical aperture placed between theoptical element and the CCD or CMOS optical detectors because the smallsize (typically 2-5 microns across) of CCD and CMOS pixels result in anautomatic restriction in the area of the detected portion 264 of thespeckle pattern 260.

In some implementations, the plurality of CCD or CMOS detectors can bein a 1×N array of either individual pixels or binned combinations ofpixels. For example, FIGS. 10A, 10B, and 10C depict a 1×S array and FIG.13 depicts a 1×4 array. Furthermore, as shown in FIG. 13, digital signalprocessing can be performed on each of the N separate digital outputs420. Each digital output 420 can contain information on the modulationof the optical pattern in a different detected portion 264 of thespeckle pattern 260 observed by each optical detection region 244. Eachdigital signal processing analysis can provide a real-time assessment ofthe modulation rate (analogous to dE/dt) in one of the detectionregions, and can be used to determine the maximum modulation rate duringeach arterial pulse. The N measurements can be averaged for eacharterial pulse to produce a more reliable estimate of the pulseamplitudes and of the pulse amplitude envelope.

In implementations using a CCD or CMOS optical detector 240 or 242(either as a single optical detector or as a plurality of detectors), anaverage optical detector output level can be set and defined as a“threshold”. The individual detector signals can be measuredsufficiently often (typically 100-2000 times per second) to resolve thespeckle pattern modulation. The actual data rate can be dependent on thecharacteristic speckle size relative to the detector area(s) and therate of movement of the optical element in relation to the light source.Each threshold crossing, defined as an occurrence where the differencebetween a detector output measurement and the threshold is opposite inpolarity from that of the subsequent detector measurement and thethreshold, can correspond to a “zero-crossing”. The threshold crossingscan be counted and analyzed in a manner equivalent to the zero-crossingcounts described above.

In some implementations, a digital signal processor (DSP) can be used toanalyze the output from one or more optical detectors 240 or 244.Various digital signal processing analysis methods can be applied todetermine the modulation rates, including, but not limited to, FastFourier Transforms (FFT), autocorrelations, and threshold crossings ofthe digital CCD or CMOS outputs.

In FFT analysis, a signal can be analyzed to determine a mean frequencyby the following algorithm:<ω>=∫ω·G(ω)dω,where ω is the angular frequency, G(ω) is the power spectrum, and ∫(ω)dωis normalized to a value of 1.G(ω) is determined by the well known convolution:G(ω)=[∫g(t)·exp(−jωt)dt] ²,where g(t) is the time varying signal, or optical detector output E inthis case. During each arterial pulse, the value of <ω> can rise andfall in proportion to the signal dE/dt described earlier. Therefore avalue of <ω>_(max) can indicate the maximum modulation rate within agiven arterial pulse cycle, and can be scaled and used to generate apulse amplitude envelope for use in determining the systolic, diastolic,and mean arterial pressures.

In some implementations, an autocorrelation method can be used in orderto determine the pulse amplitudes and pulse amplitude envelope. Inautocorrelation, the signal can be self-correlated according to therelationship:<G(τ)>=∫g(t)·g(t−τ)dt,where G(τ) is the autocorrelation function at time delay=τ, and g(t) isthe time varying signal. The value of G(0) is equal to the mean squareof the signal amplitude. The frequency spectrum is simply a convolutionof the autocorrelation function, such that:G(ω)=(1/2π)·∫G(τ)·exp(−jωτ)dτ.The determination of the mean frequency of a time varying signal usingan autocorrelation method has been described previously and is notpresented in further detail here. This calculation of G(ω) is used tocalculate the mean frequency according to the same formula as in FFTanalysis:<ω>=∫ω·G(ω)dω

In some implementations, the maximum value of dE/dt can be calculatedfor each arterial pulse during a time interval when the pressure in theblood pressure cuff is steadily decreased from a level above systolicpressure where the arterial pulse is absent. The onset of each pulse isdetected during the time interval by measuring and recording theperiodic increase of dE/dt. For each pulse, the maximum value of dE/dt(dE/dt_(max)) can be recorded as a dimensionless number, and the cuffpressure can also recorded so as to allow for the creation of anenvelope of pulse amplitudes in which the ordinate of the chart isdE/dt_(max) instead of oscillation amplitude in mmHg. An algorithm canbe applied to this envelope to determine the systolic, diastolic, pulse,and/or mean arterial pressures.

In some implementations, the zero-crossing count of the AC coupledoptical detector output can be tallied for each arterial pulse during atime interval when the pressure in an inflatable cuff 120 is steadilydecreased from a level above systolic pressure where the arterial pulseis absent. A series of arterial pulses can be detected during the timeinterval, and for each pulse the zero-crossing count can be measured andrecorded. For each pulse, the count (or average of the two countscorresponding to the rise and fall of the arterial pulse) can berecorded, and the cuff pressure can also be recorded so as to allow forthe creation of an envelope of pulse amplitudes in which the ordinate ofthe chart is the zero-crossing count instead of oscillation amplitude inmmHg. An algorithm can be applied to this envelope to determine thesystolic, diastolic, pulse and/or mean arterial pressures.

In some implementations, the time interval between pulses can bemeasured during a series of detected arterial pulses and used todetermine heart rate.

In some implementations, as the cuff pressure is decreased, the systolicpressure can be determined to be an inflatable cuff 120 pressure atwhich the first evidence of modulation of the speckle pattern occurs(i.e., the rise of the zero-crossing rate above zero, or the firstappearance of a non-zero value for dE/dt). In some implementations, thediastolic pressure can be determined to be an inflatable cuff 120pressure at which a predetermined characteristic of the modulation ofthe speckle pattern occurs. For example, the last detected arterialpulse, where the zero-crossing rate last has a non-zero value, or wherethe last non-zero value for dE/dt occurs and after which dE/dt remainsat zero while the cuff pressure is further decreased, may be taken asthe diastolic pressure. Or the appearance of the first arterial pulse ina sequence of declining arterial pulses where the value of dE/dt_(max)is 50% of the maximum value of dE/dt_(max) (i.e., the highest point onthe envelope of pulse amplitudes). In some implementations, the meanarterial pressure can be determined to be an inflatable cuff 120pressure corresponding to the arterial pulse event at which the maximumzero-crossing count or the maximum value of dE/dt_(max) occurs (i.e.,the highest point on the envelope of pulse amplitudes).

In some implementations, the systolic pressure can be calculated to beat some pressure below the cuff pressure at which the first evidence ofmodulation of the speckle pattern occurs during cuff deflation, based onan empirically determined algorithm that calculates the contribution ofsome amount of artifact in the arterial pulses acting against theoptical motion sensor 104, together with other artifact related to theelectrical noise and to the modulation of the speckle pattern.

In some implementations, the diastolic pressure can be calculated assome pressure above the cuff pressure at which a predeterminedcharacteristic of modulation of the speckle pattern occurs, based on acorresponding algorithm that calculates the contribution of artifactfrom the arterial pulses acting against the optical motion sensor 104,and other artifact.

In some implementations, a baseline measurement of blood pressuremeasurement is determined (the “Baseline”) and subsequent blood pressuremeasurements are estimated based upon a continuous monitoring of a vitalsign. For example, the baseline blood pressure reading can be obtainedusing the relative pulse amplitudes of a series of pulses obtained bymeasurement of dE/dt_(max) or the zero-crossing count as describedabove, and using either one optical detector 240, a plurality of opticaldetection regions 244, a CCD sensor array, or a CMOS sensor array. Thenthe occluding device 102 can then be adjusted to a pressure level with aknown (by virtue of said measurement of blood pressure alreadyperformed) pulse amplitude (the “Reference Amplitude”), and the arterialpulse amplitude can be measured continuously and compared to thereference amplitude. Any subsequent pulse amplitude measurement thatdiffers from the reference amplitude can be used, with a suitablealgorithm, to quantitatively measure blood pressure changes relative tothe baseline. In this implementation, the method's primary purpose iscontinuous or periodic monitoring of blood pressure changes relative toa Baseline value. In some implementations, the Baseline blood pressuremeasurement can be determined by other standard methods, such as theauscultatory method.

In some implementations, a pulse waveform morphology can be determinedby measuring the time-varying value of dE/dt. The morphology of thepulse waveform can be represented by the curve of dE/dt versus time overthe course of an arterial pulse. Alternatively the time varyingzero-crossing rate may be used, or the threshold-crossing rate in adigital CCD or CMOS detection system.

In some implementations, such as shown in FIGS. 14A, 14B, and 14C, theoutput unit 106 can determine a vital sign by one or more of the abovedescribed techniques. For example, the output unit 106 can determine anamplitude, a magnitude and/or a waveform of one or more arterial pulsesin a waveform generator 436. In some implementations, the output unit106 can include a systolic pressure waveform detector to determine asystolic pressure for a subject based upon a determined amplitude,magnitude and/or waveform and a pressure applied to the subject, whichcan be detected (e.g., a pressure detected in an inflatable cuff by apressure sensor). In some implementations, the output unit 106 caninclude a diastolic pressure calculator to determine a diastolicpressure for a subject based upon a determined amplitude, magnitudeand/or waveform and a pressure applied to the subject, which can bedetected (e.g., a pressure detected in an inflatable cuff by a pressuresensor 128). In some implementations, a heart rate calculator 446 candetermine a heart rate from either a determined arterial pulse waveformfrom the optical signal or from pressures detected in an inflatable cuffby a pressure sensor 128. In some implementations, the output unit 106can include a pulse wave timing detector 434, which can ensure that eacharterial pulse detected by the motion sensor 104 corresponds to a pulsedetected by an inflatable cuff pressure sensor 128. In someimplementations, the pulse wave timing detector 434 provides data to thewaveform generators 436 to ensure that each waveform generator 436determines a waveform consistent with pulses detected by an inflatablecuff pressure sensor 128.

In some implementations, such as shown in FIG. 14C, the output unit 106can determine an amplitude, a magnitude and/or a waveform of one or morearterial pulses for each optical detection region 244 in a series ofwaveform generators 436. In some implementations, the output unit 106can include a waveform comparator 438 to compare the plurality ofamplitudes, magnitudes, and/or waveforms. The waveform comparator 438can select the better optical detection regions 244, average the signalsfrom two or more of the optical detection regions, or otherwise computea single amplitude, magnitude, and/or waveform based on the data fromthe plurality of optical detection regions 244. In some implementations,a heart rate calculator 446 can determine a heart rate from either asingle waveform from the waveform comparator 438 from the optical signalor from pressures detected in an inflatable cuff by a pressure sensor128.

Other Motion Sensing Techniques

A variety of methods can be used to detect the amount of motion of thesensor pad 232. As described above, an amount of motion can be detectedby optical modulation techniques. In other implementations, the sensorpad 232 can be connected to a spring loaded plunger and the amount ofdisplacement of the plunger can be determined by electrical signals. Forexample, the displacement of the plunger could alter a series ofelectrical connections to indicate the position of the sensor pad 232relative to the housing 200. In some implementations, the motion sensor102 can include a shaft coupled to the sensor pad 232, such that themotion of the sensor pad 232 moves the shaft through a solenoid andcreates an electrical signal proportional to its displacement. In someimplementations, the motion sensor 102 can include an opticaltransmitter and receiver such that the optical transmitter (e.g., a LEDor VCSEL) directs light at the sensor pad 232 and the optical receiver(e.g., a photodiode) collects a portion of the light reflectedtherefrom.

In some implementations, the motion sensor can include a lever arm 610that drives a coil spring 615, as shown in FIG. 15A. The lever arm 610can be a rigid member. The lever arm 610 and the coil spring 615 can actas the return element 234 and be used to detect the amount of motion ofthe sensor pad 232. In these implementations, the housing 200 and thesensor pad 232 can be configured in a way similar to that describedabove. The opposite end of the lever arm 610 can be attached to the coilspring 615. One end of the spring coil 615 can be attached to thehousing 200 and the other end attached to the lever arm 610, with theaxis of the coil spring 615 perpendicular to the lever arm 610. In someimplementations, the coil spring 615 can be connected directly to theside of the housing (as shown in FIG. 15A) or can be attached via anadditional rigid member. The lever arm 610 can rotate (slightly) aboutan axle or axis, such rotation being constrained by a coil spring 615that is fixed to the housing 200. Action of the arterial pulse causescoiling of the coil spring 615. The degree of motion of the sensor pad232 can correlate to the degree of coiling of the coil spring 615, whichcan be measured in a number of ways. For example, the degree of coilingof the coil spring 615 can be measured by a strain gage 620. A straingage 620 can be attached to the coil spring 615 to measure themodulating strain of the coil spring 615, which would indicate theextent of coiling, and therefore the extent of motion of the sensor pad232. In some implementations, a variable resistor 625 (e.g., apotentiometer) can be used to measure the degree of coiling of the coilspring 615. For example, a shaft 630 of the variable resistor 625 can beattached to the coil spring 615 such that the shaft of the variableresistor rotates as the coil spring coils. For example, the shaft 630 ofthe variable resistor 625 can be positioned along the axis of the coilspring 615. The place of attachment of the shaft to the coil spring 615can impact the amount of rotation detected by the variable resistor. Insome implementations, the shaft of the variable resistor can be attachedto the end of the coil spring 615 attached to the lever arm 610. As theshaft of the variable resistor rotates back and forth, the amount ofresistance in the variable resistor varies. For example, the variableresistor can include a moving “wiper” 635 that moves inside thepotentiometer 640 to vary the amount of resistance. This variableresistance can be measured electronically to detect an amount of coilingof the coil spring and hence the movement of the sensor pad 232. Forexample, a constant voltage can be applied across the variable resistorand the voltage measured 645 at the “wiper” would indicate the degree ofrotation of the variable resistor shaft. As the shaft rotates back andforth, moving the “wiper” 635 in the potentiometer 640, the outputvoltage signal will vary to indicate the degree of rotation and hencethe amount of movement of the sensor pad 232.

Referring to FIG. 15B, the motion sensor can include a strain gage 620to measure an amount of flexing in the return element 234. In someimplementations, the return element 234 can be a spring. The sensor pad232 can be attached to a spring type return element 234 and the flexingof the return element 234 can be measured by a strain gage attached tothe return element 234. The detected amount of strain can directlycorrelate to the amount of flexing of the spring type return element 234and therefore to movement of the sensor pad 232. The strain gagemeasures the deflection of the spring, rather than the amount ofpressure applied to the sensor pad 232.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications can be made without departingfrom the spirit and scope of the invention. Accordingly, otherimplementations are within the scope of the following claims.

1. A vital sign measurement device comprising: an occluding deviceadapted to be placed against an anatomical location of a subject, withinwhich is an artery, and to apply a pressure to the anatomical locationof the subject to occlude the artery; a motion sensor positioned withrespect to the occluding device to sense movement corresponding to anarterial pulse when the occluding device occludes the anatomicallocation of the subject, the motion sensor including a sensor padpositioned for placement against an anatomical location of a subject andto move in response to an arterial pulse; and an output unit thatreceives, from the motion sensor, an input indicative of the amount ofmovement of the sensor pad and that generates, using the input, ameasure of the vital sign.
 2. The vital sign measurement device of claim1, wherein the occluding device is an inflatable cuff.
 3. The vital signmeasurement device of claim 1, further comprising a pressure sensor todetect a pressure applied to the anatomical location by the occludingdevice, wherein the output unit receives, from the pressure sensor, apressure input indicative of the pressure applied to the anatomicallocation, wherein the output unit generates the vital sign using theinput from the motion sensor and the pressure input.
 4. The vital signmeasurement device of claim 1, wherein the anatomical location of thesubject the body is an upper arm, and the occluding device is configuredso that the motion sensor is positionable to sense movement due to apulse of a brachial artery.
 5. The vital sign measurement device ofclaim 1, wherein the anatomical location of the subject is a wrist, andthe occluding device is configured so that the motion sensor ispositionable to sense movement due to a pulse of a radial artery.
 6. Thevital sign measurement device of claim 1, wherein the anatomicallocation of the subject is an ankle, and the occluding device isconfigured so that the motion sensor is positionable to sense movementdue to a pulse of one or more arteries in the ankle.
 7. The vital signmeasurement device of claim 1, wherein the motion sensor is an opticalsensing system comprising an optical source, an optical refractor, andan optical detector, the optical sensing system sensing an amount ofmovement from the movement, bending, or compression of at least oneportion of the optical sensing system relative to other portions of theoptical sensing system resulting in a change in an optical signalreceived by the optical detector.
 8. The vital sign measurement deviceof claim 1, wherein the motion sensor includes a shaft connected to thesensor pad and a solenoid, the shaft moving through the solenoid tocreate an electrical signal proportional to the movement of the shaft inresponse to the movement of the sensor pad.
 9. The vital signmeasurement device of claim 1, wherein the motion sensor includes apotentiometer to detect an amount of movement of the sensor pad.
 10. Thevital sign measurement device of claim 1, wherein the motion sensorincludes a return element attached to the sensor pad to counter a forcefrom the arterial pulse and to return the sensor pad to an initial stateafter the arterial pulse.
 11. The vital sign measurement device of claim10, wherein the return element comprises a spring.
 12. The vital signmeasurement device of claim 11, wherein the motion sensor comprises astrain gauge adapted to detect an amount of strain in the spring. 13.The vital sign measurement device of claim 10, wherein the motion sensoris adapted such that an applied pressure of 150 mmHg will displace thesensor pad by at least 1 mm from a resting state.
 14. The vital signmeasurement device of claim 13, wherein the motion sensor is adaptedsuch that an applied pressure of 150 mmHg will displace the sensor padby at least 2 mm from the resting state.
 15. The vital sign measurementdevice of claim 10, wherein the motion sensor further comprises ahousing, wherein an upper surface of the sensor pad is approximatelyflush with an upper surface of the housing when a pressure of between 80and 150 mmHg is applied to the sensor pad.
 16. The vital signmeasurement device of claim 15, wherein the upper surface of the sensorpad is approximately flush with the upper surface of the housing when apressure of between 100 and 130 mmHg is applied to the sensor pad. 17.The vital sign measurement device of claim 1, wherein the vital sign isat least one of a heart rate, an arterial pulse waveform, a systolicblood pressure, a diastolic blood pressure, a mean arterial bloodpressure, a pulse pressure, and an arterial compliance.
 18. The vitalsign measurement device of claim 1, further comprising a display todepict a vital sign measurement generated by the output unit.
 19. Thevital sign measurement device of claim 1, further comprising an alarmsystem to produce a human detectable signal when a vital signmeasurement generated by the output unit meets a predetermined criteria.20. The vital sign measurement device of claim 1, wherein the motionsensor and the output unit are adapted to sense a pulse amplitude of thearterial pulse from the displacement of the sensor pad.
 21. A method ofmeasuring a vital sign of a subject, the method comprising: placing anoccluding device against an anatomical location of a subject, withinwhich is an artery, and applying a pressure to the anatomical locationof the subject with the occluding device to occlude the artery, theoccluding device holding a motion sensor having a sensor pad; reducingthe pressure applied to the anatomical location of the subject; sensingmovement of the sensor pad corresponding to at least one arterial pulse;and generating a measure of the vital sign using an input indicative ofthe amount of sensed movement of the sensor pad.
 22. The method of claim21, wherein the vital sign is a systolic blood pressure, wherein themeasure of the systolic blood pressure corresponds to a pressure appliedto the anatomical location of the subject by the occluding device at thetime of a first sensed movement of the sensor pad as the pressureapplied to the anatomical location of the subject is reduced from apressure fully occluding the artery.